Support log-level as an environment variable

When working on consul-template I find myself switching between log levels quite a bit and having to keep editing scripts or commands. If it supported an environment variable for setting the log-level it would make this much easier.

The cli argument should override the environment variables.

My initial proposed variable name is `CONSULT_TEMPLATE_LOG_LEVEL`, though that is a bit long.

Start by tracing the CLI log-level argument and the configuration path that determines the active log level. Check how environment variables are loaded, then verify that the CLI argument overrides CONSULT_TEMPLATE_LOG_LEVEL or the selected variable name; done means log-level can be set from the environment without editing commands and the precedence is tested.

Written by the indexing model from the issue text.